Not Applicable
1. Field of the Invention
The current invention relates to selective cooling, or hypothermia, of an organ, such as the brain, by cooling the blood flowing into the organ. This cooling can protect the tissue from injury caused by anoxia or trauma.
2. Background Information
Organs of the human body, such as the brain, kidney, and heart, are maintained at a constant temperature of approximately 37xc2x0 C. Cooling of organs below 35xc2x0 C is known to provide cellular protection from anoxic damage caused by a disruption of blood supply, or by trauma. Cooling can also reduce swelling associated with these injuries.
Hypothermia is currently utilized in medicine and is sometimes performed to protect the brain from injury. Cooling of the brain is generally accomplished through whole body cooling to create a condition of total body hypothermia in the range of 20xc2x0 to 30xc2x0 C. This cooling is accomplished by immersing the patient in ice, by using cooling blankets, or by cooling the blood flowing externally through a cardiopulmonary bypass machine.
Total body hypothermia to provide organ protection has a number of drawbacks. First, it creates cardiovascular problems, such as cardiac arrhythmias, reduced cardiac output, and increased systemic vascular resistance. These side effects can result in organ damage. These side effects are believed to be caused reflexively in response to the reduction in core body temperature. Second, total body hypothermia is difficult to administer. Immersing a patient in ice water clearly has its associated problems. Placement on cardiopulmonary bypass requires surgical intervention and specialists to operate the machine, and it is associated with a number of complications including bleeding and volume overload. Third, the time required to reduce the body temperature and the organ temperature is prolonged. Minimizing the time between injury and the onset of cooling has been shown to produce better clinical outcomes.
Some physicians have immersed the patient""s head in ice to provide brain cooling. There are also cooling helmets, or head gear, to perform the same. This approach suffers from the problems of slow cool down and poor temperature control due to the temperature gradient that must be established externally to internally. It has also been shown that complications associated with total body cooling, such as arrhythmia and decreased cardiac output, can also be caused by cooling of the face and head only.
Selective organ hypothermia has been studied by Schwartz, et. al. Utilizing baboons, blood was circulated and cooled externally from the body via the femoral artery and returned to the body through the carotid artery. This study showed that the brain could be selectively cooled to temperatures of 20xc2x0 C. without reducing the temperature of the entire body. Subsequently, cardiovascular complications associated with total body hypothermia did not occur. However, external circulation of the blood for cooling is not a practical approach for the treatment of humans. The risks of infection, bleeding, and fluid imbalance are great. Also, at least two arterial vessels must be punctured and cannulated. Further, percutaneous cannulation of the carotid artery is very difficult and potentially fatal, due to the associated arterial wall trauma. Also, this method could not be used to cool organs such as the kidneys, where the renal arteries cannot be directly cannulated percutaneously.
Selective organ hypothermia has also been attempted by perfusing the organ with a cold solution, such as saline or perflourocarbons. This is commonly done to protect the heart during heart surgery and is referred to as cardioplegia. This procedure has a number of drawbacks, including limited time of administration due to excessive volume accumulation, cost and inconvenience of maintaining the perfusate, and lack of effectiveness due to temperature dilution from the blood. Temperature dilution by the blood is a particular problem in high blood flow organs such as the brain. For cardioplegia, the blood flow to the heart is minimized, and therefore this effect is minimized.
Intravascular, selective organ hypothermia, created by cooling the blood flowing into the organ, is the ideal method. First, because only the target organ is cooled, complications associated with total body hypothermia are avoided. Second, because the blood is cooled intravascularly, or in situ, problems associated with external circulation of blood are eliminated. Third, only a single puncture and arterial vessel cannulation is required, and it can be performed at an easily accessible artery such as the femoral, subclavian, or brachial. Fourth, cold perfusate solutions are not required, thus eliminating problems with excessive fluid accumulation. This also eliminates the time, cost, and handling issues associated with providing and maintaining cold perfusate solution. Fifth, rapid cooling can be achieved. Sixth, precise temperature control is possible.
The important factor related to catheter development for selective organ hypothermia is the small size of the typical feeding artery, and the need to prevent a significant reduction in blood flow when the catheter is placed in the artery. A significant reduction in blood flow would result in ischemic organ damage. While the diameter of the major vessels of the body, such as the vena cava and aorta, are as large as 15 to 20 mm., the diameter of the feeding artery of an organ is typically only 4.0 to 8.0 mm. Thus, a catheter residing in one of these arteries cannot be much larger than 2.0 to 3.0 mm. in outside diameter. The small size of the feeding artery also limits the size and type of heat transfer element that can safely be used.
A catheter based on the circulation of water or saline operates on the principle of transferring heat from the blood to raise the temperature of the water. Therefore, it is essential to use a heat transfer element that transfers heat from the blood to the cooling fluid as efficiently as possible, while restricting the flow of blood as little as possible. So, it would be beneficial to have a heat transfer apparatus that can be inserted percutaneously into an artery of restricted size, that can efficiently transfer heat, and that will not significantly limit the flow rate of blood in the artery during application of cooling.
The present invention is a cooling apparatus comprising a flexible catheter which can be inserted through the vascular system of a patient to a feeding artery, with an inflatable balloon heat exchanger near the distal end of the catheter. The present invention also encompasses a method for using such a device to perform selective organ cooling. After placement in the selected feeding artery, the heat exchanger balloon is inflated by pressurization with a saline solution, via a supply lumen in the catheter. The heat exchanger balloon has one or more blood passageways passing through it, from a proximal aspect of the balloon to a distal aspect of the balloon. When the heat exchanger balloon is inflated to contact the wall of the artery in which it is placed, each of the blood passageways comprises a tube having an inlet in one face of the heat exchanger balloon and an outlet in another face of the heat exchanger balloon, thereby allowing blood to continue flowing through the artery after inflation of the balloon. The blood passageway tubes can be constructed of a material having a relatively high thermal conductivity, such as a thin metallized polymer, such as a film with one or more metallized surfaces. Alternatively, the blood passageway tubes can be constructed of a metal-loaded polymer film. Further, the entire heat exchanger balloon can be constructed of such a material, in order to maximize the cooling capacity of the heat exchanger.
After inflation of the heat exchanger balloon, the saline solution, which is chilled by an external chiller, continues circulating through the interior of the heat exchanger balloon, around the blood passageway tubes, and back out of the balloon through a return lumen in the catheter. This cools the blood passageway tubes, which in turn cool the blood flowing through them. This cooled blood then flows through the selected organ and cools the organ.
The device can also incorporate a lumen for a guidewire, facilitating the navigation of the catheter through the vascular system of the patient.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: